Extend slices and accessors for arrays

Things like

a = [1, 2, 3]
b = a[1:2]  # [2]
c = [[1, 2], [3, 4]]
d = c[0]  # [1, 2]

The slicing works but the propety reference (via accessors) does not:

Type error: None:4:5 --> c[0] <--
Invalid use of index in type IntArray

Also this should be possible: e = c[0][1] (should be 2).

The grammar is already adapted. Only the interpreter code and tests have to be written.